As the longest presidential campaign in history enters its final month, neither candidate has yet delivered a telling blow to the glass jaw of the other. Jimmy Carter has failed to make Ronald Reagan the issue, and Ronald Reagan has failed to rally the country against the president's dismal performance on inflation. Both men now work from weak positions, and the likelihood is that the campaign will play through to the bitter end as a national embarrassment.

As a target, Reagan was hard to miss. He is long on age, short on experience, and even shorter on information. Worse still is what he likes to call his "basic philosophy."

In the early phases of the campaign, when he ran free as an unfettered candidate, Reagan spoke his mind. He cast doubts on Darwinian evolution, embraced the war in Vietnam and expressed the willingness to equate Taiwan with mainland China. Those comments were put down as bloopers.

In fact, they derive from a view of the world. It is a view that pits God-fearing folk against unbelievers, aggressors against victims, commies against true compatriots. By instinct, in other words, Reagan devised the world into good guys and bad. It is an appealing quality. But not ideal for an office that mainly demands a strong sense of priorities, a capacity to deal with neutral conflicts, to balance fairly between competing goods.

That self-evident point was not made by the Democrats largely because of their constituency and their leader. The component elements of the Democratic Party -- labor, the minorities, middleclass professionals, women and young people -- have no instinctive affection for Jimmy Carter. To win their support against Reagan he had to scare them. Personally, moreover, Carter has a mean and petty streak -- particularly when running for office.

So when the president moved in on Reagan, he overdid it. He implied Reagan was a racist, and a menace to peace, and to economic security. Everybody sensed the unfairness of those shots. Thus Reagan was able to weather the assault. Now he has gone through the fire, and to his credit he has moderated his language. From here on in, the Democrats will not find it easy to make him the issue.

The Republicans missed their mark in much the same fashion. The Carter administration has put forward three different economic programs in the past eight months. The Consumer Price Index has risen to a record high for peacetime, dropped, and is now beginning to bounce back. The core rate of inflation -- the rate that goes on continually from year to year -- has risen during the Carter administration from around 6 percent to 10 percent. As a result, economic growth is slow, and recovery tends to self-destruct. New bursts of inflation set in at ever higher levels of unemployment. Moreover, in its eagerness to prmote recovery, the Carter administration leaves the task of sitting on inflation to the Federal Reserve Board. That inevitably means higher interest rates. But when the rates rise, the president and the secretary of the Treasury criticize the Fed in public. They thus inspire doubts among the foreign holders of the dollar, who then become the final arbiters of American economic policy.

A serious Republican candidate would have made the economy issue No. 1, 2 and 3. But Reagan got himself tangled up in the minutiae of controversy connected with the Laffer Curve and the Kemp-Roth proposals for tax reduction. He has not yet emerged from those thickets of obscurity. Meanwhile, the most recent figures for wholesale prices, unemployment and money supply work somewhat in the president's favor. It is now too late to make the economic issue.

Not surprisingly, given that kind of a campaign, the two candidates remain in close contention. The Reagan people claim they are a little ahead. The Carter people acknowledge that they're a little behind.

But nobody knows which way the declining vote for John Anderson will break. Reagan has no strong cards yet to play. Carter has the stamina for a blitz finish, and the last-minute organizational support that labor and other allies can muster. There remains the possibility of an October surprise, especially in foreign policy.

The strong impression in this quarter is that the election continues to be up for grabs. But while the winner is in doubt, the mandate will surely be muddy. For the most likely prospect is that the biggest number of Americans -- probably more than 80 million -- will be those who stay away from the polls.